Methods and devices to test for a leak in fuel tank systems have been known for a long time in varying forms of embodiment. In most of them, an overpressure or a vacuum is introduced into the fuel tank system, which is cut off, and on the basis of the pressure gradient, which builds up in the process, a leak-proof or leaky fuel tank system is suggested. Methods of this kind proceed, for example, from the German patent DE 196 36 431 A1.
A method and a device to determine leakages in the fuel supply system of a motor vehicle are known from the European patent EP 0 952 332 B1, wherein the leak diagnosis results from an analysis of a signal of a differential pressure sensor. By means of this analysis, influencing factors, such as the fuel tank fill level and the fuel outgassing, can be taken into consideration.
During the SAE-High Tech Conference in Indianapolis in 1999, a method referred to as the “natural vacuum leakage detection (NVLD)” was introduced, wherein the fuel tank is cut off for a specified period of time by means of a cutoff device. When the tank is leak-proof, a vacuum results thereby. Provision is made in this method for a vacuum switch, which changes its switching status starting at a certain threshold value. This switching status is acquired. If no switching action is acquired within a specific time span, the presence of a leak is suggested. The idea behind this method consists of utilizing the vacuum, which normally occurs when the motor vehicle is turned off and the tank is cut off, to test the leak tightness of the fuel tank system. It is thereby assumed that the vacuum results from a cooling down of the fuel tank.
A method became known from the patent WO 01/69073 A1; wherein during the overrun, i.e. when the vehicle has just been turned off, a check valve is closed, and during the overpressure phase, which initially takes place subsequent to the valve closing, the overpressure is measured. If this overpressure no longer increases, the check valve is again opened and is closed again after a specified time span. A vacuum phase takes place immediately subsequent to this, wherein the vacuum is measured or is ascertained in another way. After completion of both of these phases, the leak tightness of the fuel tank system is suggested from the overpressure and the vacuum. This method is disadvantageous; in that during both of the phases, during the vacuum phase as well as during the overpressure phase, the check valve has to be repeatedly opened and closed; and in the this time period an acquisition of the overpressure and the vacuum also has to take place. That requires that corresponding measuring devices are employed during these phases. In so doing, the battery of the motor vehicle is significantly stressed.